Toot toot! All aboard the Jacobs Express

Picture a ravishing midnight-blue steam engine with gilt lettering, wood panelling and a strange fuzzy phallus on a boom thrusting through the window until it eventually comes to rest level with Marc Jacobs's nose. We were on the Vuitton Express - Jacobs and me, plus the keeper of the flame (a member of staff whose job it is to light his cigarettes and hold them until he's finished the previous one). Oh and did I mention that Catherine Deneuve, Sarah Jessica Parker, Harvey Weinstein and the model Natalia Vodianova (who announced that Jacobs's collection had just given her a fashion orgasm) were also on board, patiently waiting for Jacobs to finish his disquisition with The Daily Telegraph so that they could pay homage? And did I say that Jacobs was wearing a black stretch, calf-length dress, accessorised with diamanté buckled shoes. Ho hum. Just another day at work.

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The train - hand-built by the Vuitton team for their fashion show on Wednesday at a cost of who knows how much - had puffed in to platform Fashion: a replica of a turn-of-the-century Parisian railway station that had also been constructed for the show. Fifteen minutes earlier, a carriage full of models in fin de siècle tailoring had alighted to take a turn on the catwalk, trailed by Vuitton-liveried porters festooned with jewelled and mink-trimmed luggage. Now the train was being deployed as Jacobs's post-show sanctum. "The heart of the collection," he said, "was really an exploration of bad taste. But in a good way. Those ochres, mustards, pinks and grid patterns - some people would consider them loud, odd, maybe even a bit ugly." Pushing the taste boundaries - it's a favourite Jacobs activity.

Meanwhile, outside, pressing against the windows of the Vuitton Express while he talked, hundreds of journalists and bloggers took pictures on their iPhones and brandished microphones. It was like being on a progress with JFK, or maybe Puff Daddy. Except we were going nowhere.

What a perfect metaphor for fashion.

I don't mean that disparagingly. Fashion has always inched forward, or backward sometimes, incrementally - except when there are cataclysmic world wars to trip it up and send it sprawling into a new era. There's only so much a designer can do, given that clothes have to accommodate the same old human body: the rest is fabric innovation, which is interesting but nerdy and, as Jacobs said, taste, which is fascinating and open to infinite interpretations.

Toying with "bad" taste has become the default game for all the most influential designers of the 21st century, from Miuccia Prada and Jacobs to Christopher Kane, who used purple moiré, that watermark-patterned satin beloved of the makers of coffin linings, in his Autumn Winter collection, because it "was gross, yet brilliant". In some ways, exploring "bad" taste is the safe option; a passport that allows the bearer to commit all sorts of transgressions, but still be allowed to pass without hindrance across planet fashion. They can royally balls-up their collections and claim it was a deliberate act of provocation. By contrast, if any designer had the temerity to say they were into good taste - or, worse still, committed to tasteful clothes - they'd be labelled an out-of-touch loser.

That's fine too. Because if fashion has any function - other than to make the likes of Jacobs rich and bring the rest of us fleeting happiness and continuous entertainment - it's to challenge our status quo.

Too much of anything becomes stultifying, even good taste, as John Betjeman argued in Ghastly Good Taste, which is both a passionate defence of Victorian and Edwardian architecture, which, when Betjeman was writing in 1933, was considered hideously beyond the pale, and a satire of the sterile commandments of modernism from which we derive our contempt for antimacassars, plastic seat covers and saving things "for best". But doesn't some of that whiff of class prejudice? The upper middle class looking down on the lower middle, just as today the squeezed middle consoles itself by sneering at the vulgar trappings of the super-rich. Perish the thought, but it's not inconceivable that some may find the Vuitton show, with its untold budget, a tad gaudy.

But what the heck is good taste anyway? Immanuel Kant acknowledged in 1790 that, while there's a formula for universal beauty, taste is subjective. Most of us know that empirically, though our hearts tell us our own taste is excellent and the standard by which all others ought to be measured. Clearly, taste is tribal. It's also subject to fashion - see Victorian and Edwardian architecture, or come to that, minimalism, which, in its diluted, Changing Rooms version, looks just as naff as plastic Louissomething.

Kant believed that fashion's appeal lay in creating social distinction. But it can also change taste and help make social distinctions more fluid. If only Kant could have been at the Vuitton show.